Successful engraftment and maintenance of hematopoietic function following bone marrow transplantation (BMT) depends on the functional capacity as well as the quantity of hematopoietic stem cells (HSC) in the graft. The cell surface CD34 antigen is expressed on early human HSC and enumeration of CD34+ cells in bone marrow and peripheral blood has been used to quantify the number of HSC contained in stem cell grafts. Previous studies have shown that the number of CD34+ HSC infused correlates well with the speed of hematologic recovery following transplant. Similarly, colony forming unit granulocyte-monocyte (CFU-GM) and long-term culture initiating cell (LTC-IC) assays have been used as functional measures of HSC and to predict early engraftment following transplant. However, despite the association of these measures with early engraftment, none of these stem cell measures correlate with the level of long-term hematopoietic function following transplant. To identify characteristics of stem cell grafts associated with long-term hematopoietic reconstitution, I have examined short and long term engraftment in 119 patients with B cell lymphoma who received autologous BMT. As expected, CD34+ cell dose and CFU-GM infused were significant predictors of early engraftment but did not predict for long-term hematopoietic reconstitution (LHR). Similarly, phenotypic stem cell subsets (CD34+CD38- and CD34+HLADR-) were associated with early engraftment but not with LHR. In contrast, time to neutrophil engraftment after transplant was found to be a sensitive predictor of LHR. These observations suggest that current phenotypic markers of HSC do not identify those cells responsible for long-term hematopoietic function in vivo. To identify phenotypic and functional characteristics of these cells, I propose to extend my preliminary analysis to larger numbers of patients who received autologous peripheral blood stem cells and to evaluate novel phenotypic and functional markers of early human HSC. The aim of these studies will be to identify patients with abnormal hematopoietic function 1 year post-transplant and to identify phenotypic markers of mobilized stem cells that are associated with abnormal long-term hematopoietic reconstitution in these patients. These studies will also identify factors that characterize poor stem cell mobilization in patients with limited numbers of harvested HSC. The overall goal of my studies is to improve the long-term clinical results of autologous HSCT in patients with hematological malignancies.